The present invention relates generally to optical code scanners and, more particularly, concerns a method and apparatus for scanning optical codes which improves the quality of the scanned image and the range of operation of the scanner.
Anyone who has shopped in a modern supermarket is familiar with optical code imaging devices, in this case a bar code scanner, which facilitate rapid checkout by scanning barcodes imprinted on product packages. This is a relatively undemanding application of barcode reading, as a package is essentially brought to a standstill by the operator for purposes of reading the bar code. However, the barcode reader still must have a reasonable range of distances of operation, since the user cannot place a barcode at precisely the same location every time.
More recently, optical code readers have been utilized in production lines where items are assembled, where they are inspected, where they are packaged, and the like. This application of optical code reading is far more demanding, as products move on a production line at a relatively high speed, for example, a conveyor belt. To avoid the creation of a bottleneck on the production line, it is therefore important that accurate decoding of optical codes take place without reducing the speed at which the objects move down the production line. The speed at which an optical code can be decoded accurately therefore becomes a primary concern. Everything else being equal, optical codes can be detected at higher production line speeds and more reliably if the code reader can scan the code over a greater range of distances from the scanner.
One form of optical scanner commonly used with linear barcodes projects a laser beam at a remote optical code and scans the beam linearly along the direction of the barcode. More of the laser beam is reflected from the light areas of the barcode than the dark areas (the bars), so the light reflected from the barcode, when sensed, contains a sequence of bright and dark portions corresponding, respectively, to the spaces and bars of the barcode, respectively.
Accurate detection of the light and dark areas of the barcode requires that a well focused light source be scanned over the barcode, particularly in the presence of substantial ambient light, to make an accurate determination which areas are light or dark and, in particular, where the transition between the light and dark areas occurs. A common approach is to focus the light source to a specific position where the barcode is expected to be and to restrict the beam diameter by passing the beam through an aperture of predefined size. FIG. 1 is a schematic representation of a beam that has been so processed. As may be seen, the beam will have a minimum diameter portion or waist and will increase in width on either side of the waist. Although the quality of scanning is excellent when the optical code is located at the waist, the quality decreases, with an attendant degradation of optical resolution, at distances away from the waist, particularly at positions closer to the scanner. As a result, the scanner has an operating range R, beyond which resolution may be unacceptable or the contrast may be reduced so much as to make the optical code unreadable. The operating range R could be increased by reducing the size of the aperture, but only at the expense of reducing overall illumination. That is, a substantial increase in the brightness of the light source becomes necessary.
Therefore, a substantial need exists in the prior art to improve the operating range of optical code scanners.